Each session discussion page is a Google Doc.
When you click on thae session link, the Google Doc document will come up on your computer screen, and you can enter your comments directly. It will look similar to a MS Word document, but it will be on the internet, rather than on your computer. And everyone in the book group will be adding comments to the same page - so anyone with a link to the page can see what you’ve entered. This is perfect to a shared discussion.
Note that in order for you to be able to add to the discussion pages, I’ve given you “editor” status. This means that you could inadvertantly delete the page. Be careful not to do that!